Method of producing fatty acid alkyl ester for diesel fuel oil

ABSTRACT

A method for producing a fatty acid alkyl ester for diesel fuel oil starting with a fat/oil material such as an edible oil whereby the qualities required of diesel fuel oil can be satisfied and the wastes from the production process can be minimized. The method is characterized by including subjecting a fat/oil material to transesterification with an alcohol, washing with water the oily phase of the reaction mixture, eliminating water after washing via absorption by a high-water-absorptive resin, and then separating the high-water-absorptive resin gel to thereby provide a fatty acid alkyl ester suitable for diesel fuel oil.

TECHNICAL FIELD

The present invention relates to a method for producing a fatty acidalkyl ester for diesel fuel oil. More particularly, the inventionrelates to a method for producing a fatty acid alkyl ester suitable foruse as a fuel for diesel automobiles or the like, the method includingproducing a fatty acid alkyl ester through transesterification of analcohol, and a plant-derived fat/oil or a waste edible oil discharged byrestaurants, food manufacturing factories, homes, etc. where theplant-derived fat/oil is consumed.

BACKGROUND ART

Generally, edible oil which has been consumed in and discharged byrestaurants, food manufacturing factories, homes, etc. (i.e., wasteedible oil) is treated in a manner of, for example, burying in soil asolidified oil formed through treatment with a solidification agent; ordischarging as domestic waste and burning. However, in response to thegrowing consciousness of clean global environment, active attempts havebeen made to effectively re-use such a waste oil. In one ongoingattempt, a fatty acid methyl ester is produced throughtransesterification with methanol, and an oil suitable for diesel fuelis produced from the ester.

Specifically, a fatty acid methyl ester produced from a vegetable oilserving as an edible oil is similar to gas oil in terms of physicalproperties, such as viscosity and specific gravity, and combustionproperties. Therefore, feasible use of the ester as bio-diesel fuelwithout the requirement to modify an engine has been already discussed.Recently, the fatty acid methyl ester has been of interest as arecyclable bio-related fuel, and wide-spread utilization thereof hasbeen started in the United States and Europe. In Europe and America,however, a fatty acid methyl ester which has been produced from freshvegetable oil is predominantly employed. Thus, since the production costof the ester is higher than that of gas oil, the ester is usuallyemployed as a mixture with gas oil.

In a fatty acid methyl ester which has been produced throughtransesterification of a fat/oil with methanol, glycerin (by-product),monoglyceride, diglyceride (reaction intermediates), and unreactedtriglyceride remain even after completion of purification. When suchresidues remain in a large amount, the ester fails to satisfy qualityrequirement for a fuel oil. Thus, the amount of the residues ispreferably reduced to as low a level as possible.

In other words, in a fatty acid methyl ester single system, hydrophilicglycerin which has been solubilized in the oily fatty acid methyl esterby the mediation of monoglyceride and diglyceride serving as amphiphilicsubstances may be separated from the oil and precipitate as a result ofchanges in external factors such as storage time and temperature. In amixture system with gas oil, monoglyceride and diglyceride are dissolvedalso in the gas oil, thereby reducing the solubilization degree ofglycerin in oil and increasing the possibility of separation andprecipitation of glycerin. If such a phenomenon occurs during storage orin a fuel piping in an automobile, the ester systems cause someproblems, failing to be employed as diesel fuel. Needless to say, theamount of methanol and water remaining in the fuel must be reduced to aslow a level as possible.

Therefore, in Europe and America, quality standards of fatty acid methylesters for use as diesel fuel oil have been determined. In accordancewith the standards such as Germany (DINE 51606), France (Journalofficial), and Italy (UNI 10635), purity of fatty acid methyl ester is98% or more, and monoglyceride content, diglyceride content,triglyceride (unreacted) content, and glycerin content are 0.8% or less,0.2 to 0.4% or less, 0.2 to 0.4% or less, and 0.02 to 0.05% or less,respectively. At present, European (EU) universal standards are underconsideration, and a most possible candidate purity of fatty acid methylester is 98% or more and glycerin content of 0.03% or less, andmonoglyceride content, diglyceride content, and triglyceride content areconsidered to be 0.8% or less, 0.4% or less, and 0.4% or less,respectively, similar to the case of the Germany standards. Asstipulated in ASTM PS-121-99 (US), glycerin content is 0.02% or less. InJapan, although no particular action has been taken for setting thestandards for fatty acid methyl esters for use as a fuel, the standardswill be discussed in the near future with reference to the Europeanstandards or other standards, so as to prevent problems in automobilesduring driving.

When fatty acid methyl ester is purified under conditions satisfying,among the aforementioned quality standards in Europe and othercountries, glycerin content of 0.03% or less (first condition) andpurity of fatty acid methyl ester of 98% or higher (second condition),the purified ester has satisfactory qualities (e.g., viscosity andspecific gravity) for use as diesel fuel oil and reduced monoglyceridecontent, diglyceride content, and triglyceride content. Even thoughthese glycerides are present in very small amounts, they may conceivablysolubilize methanol, water, or other oily impurities. Accordingly, thethus-obtained fatty acid methyl ester has substantially satisfactoryqualities.

In view of the foregoing, demand exists for a method for purifying analkyl ester for the purpose of enhancing its purity and reducingglycerin content.

A method for producing fatty acid alkyl ester from a fat/oil materialhas conventionally been known. In one exemplified method, a fat/oil(fatty acid triglyceride) is reacted with an alcohol in the presence ofacid or an alkaline substance, to thereby form a fatty acid alkyl ester,followed by purifying and washing with water so as to remove acid, analkaline substance, and other water-soluble substances. According to themethod, a mixture liquid readily forms an emulsion through washing withwater, and the emulsion must be left to stand for one day so as toremove added water.

Japanese Patent Application Laid-Open (kokai) No. 7-310090 discloses animproved purification method based on washing with water, in whichwashing water is added to a purified mixture and the resultant mixtureis heated to 70 to 90° C., thereby avoiding emulsification andaccelerating phase separation. However, in order to prevent hydrolysisof the formed fatty acid methyl ester caused by high-temperaturetreatment, the alkaline substance employed in reaction and dissolved inthe fatty acid methyl ester must be neutralized with acid. In themethod, washing effect is completely attained when 20 parts by weight ormore of washing water is added to 100 parts by weight of the fatty acidmethyl ester, and washing is performed twice. Although the method isenvisaged to lower glycerin content, the purity may be lowered throughhydrolysis caused by high-temperature treatment (i.e., although thealkaline substance is neutralized by an acid substance, correctneutralization is difficult, since the reaction system containsdifferent phases; in fact, the high-temperature treatment is performedunder alkaline or acidic condition). In addition, a large amount ofwastewater must be treated.

Regarding facilities that produce waste edible oil, waste edible oil iscollected by small amounts from a large number of facilities, andtherefore, in consideration of transfer costs and other factors (e.g.,500 to 5,000 kg-fuel/day, usually 500 to 1,000 kg-fuel/day), such wasteedible oil is reasonably treated in many small-scale facilities. Thus,installation of an additional wastewater treatment facility for carryingout washing treatment with water in each small-scale facility is verydisadvantageous in terms of economy and space.

Japanese Patent Application Laid-Open (kokai) No. 10-245586 discloses indetail some purification methods that do not involve washing with water,the purification methods being included in a method for producing adiesel fuel oil from a waste edible oil. The disclosed purificationmethods includes a purification method employing adsorption of fattyacid alkyl ester by an adsorbent such as active terra alba. Throughemployment of the method, no wastewater is produced, and an alkalinecomponent employed in the reaction can be removed via adsorption.However, glycerin, monoglyceride, diglyceride, and other substances aredifficult to remove, and the purified fatty acid alkyl ester hasinsufficient quality for serving as a diesel fuel oil.

As described above, no conventional methods for purifying fatty acidalkyl ester for diesel fuel oil satisfy qualities required of dieselfuel oil and minimization of the amounts of wastes from the productionprocess. Particularly, minimization of the amounts of wastes is acritical issue, when “zero-emission of waste” is to be attained, to amaximum degree, in a large number of small-scale facilities employingwaste edible oil as a raw material.

DISCLOSURE OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide a method for producing a fatty acid alkyl ester for diesel fueloil employing as a starting material a fat/oil material such as anedible oil, particularly waste edible oil, whereby the qualitiesrequired of diesel fuel oil can be satisfied and the wastes from theproduction process can be minimized.

The present inventors have carried out extensive studies so as to attainthe aforementioned object, and have found that a fatty acid alkyl esterhaving qualities sufficient for serving as diesel fuel oil can beproduced through a simple, small-scale process in which, in the courseof collection of a fatty acid alkyl ester through purification based onwashing with water a fatty acid alkyl ester reaction mixture obtainedthrough transesterification of a fat/oil material such as an edible oil,the amount of water for washing can be remarkably reduced throughemployment of washing an emulsion-state mixture with water and use ofhigh-water-absorptive resin, whereby the amount of wastes such as washliquid can be minimized. The present invention has been accomplished onthe basis of this finding.

Accordingly, the present invention is directed to a method for producinga fatty acid alkyl ester for diesel fuel, including transesterificationof fat/oil and alcohol, removing excessive alcohol after completion ofthe transesterification, separating a heavy liquid predominantlycontaining glycerin, to thereby produce a light liquid predominantlycontaining a fatty acid alkyl ester, and treatment of the light liquid,

wherein the treatment of the light liquid comprises

a first step of washing the light liquid with added water;

a second step of causing wash liquid to be absorbed byhigh-water-absorptive resin; and

a third step of removing the high-water-absorptive resin in the form ofgel which has absorbed the wash liquid in the second step.

The third removal step is generally performed through filtration orcentrifugal separation.

The aforementioned first and second steps may be carried outsequentially or simultaneously. The two steps may be carried outsimultaneously with the aforementioned third step.

In the first step, water for washing is preferably used in an amount of1 to 10 parts by weight based on 100 parts by weight of the lightliquid. In the second step, the high-water-absorptive resin ispreferably used in an amount of 1 to 20 parts by weight based on 100parts by weight of the water for washing.

When a fatty acid methyl ester—most preferred as a fuel—is a targetester product, the following more specific production conditions arepreferably employed. Namely, 100 parts by weight of a fat/oil materialand 10 to 30 parts by weight of methyl alcohol are subjected to theaforementioned transesterification in the presence of 0.2 to 1.5 partsby weight of added potassium hydroxide serving as a catalyst, and thereaction is performed at 50 to 100° C. Subsequently, a light liquidobtained from the reaction mixture is further treated.

In the production of a fatty acid alkyl ester for diesel fuel throughtreatment of a light liquid obtained from a transesterification mixtureproduced from a fat/oil and an alcohol, a fatty acid alkyl ester ofsatisfactory quality can be produced through the method for treating alight liquid employed in the present invention, and the productionprocess requires no waste water treatment. Particularly when a wasteedible oil is employed as a raw material, no requirement of waste watertreatment apparatus—a heavy apparatus—is a great advantage for a largenumber of small-scale facilities.

THE MOST PREFERRED EMBODIMENT TO CARRY OUT THE INVENTION

Embodiments for carrying out the preset invention will next be describedin detail.

<Transesterification>

In the production of a fatty acid alkyl ester for diesel fuel accordingto the present invention, examples of the fat/oil serving as a startingmaterial of transesterification between the fat/oil and an alcoholincludes rapeseed oil, sesame oil, soybean oil, corn oil, sunflower oil,palm oil, palm kernel oil, coconut oil, safflower oil, and mixtures oftwo or more species thereof.

In other words, the fat/oil is a mixture of fatty acid triglyceridescontaining an unsaturated or saturated aliphatic alkyl group havingabout 8 to 22 carbon atoms. Preferably, a fat/oil serving as a rawmaterial for producing a fatty acid alkyl ester for diesel oilpredominantly contains fatty acid triglycerides which form a fatty acidalkyl ester in the form of liquid and which has an unsaturated orsaturated aliphatic alkyl group having about 10 to 18 carbon atoms.Particularly preferred fat/oils contain predominantly fatty acidtriglycerides containing an unsaturated or saturated aliphatic alkylgroup having about 12 to 18 carbon atoms.

Therefore, rapeseed oil, sesame oil, soybean oil, corn oil, palm oil,and mixtures of two or more species thereof are particularly preferablyemployed.

No particular limitation is imposed on the types of these fat/oilmaterials, and fresh (not used) edible oil and waste edible oil may beemployed. In the present invention, use of a waste edible oil as a rawmaterial is particularly advantageous, from the viewpoint of economy andsocial demand.

Waste edible oils have a variety of compositions and properties,depending on their origins. Generally, differing from fresh edible oil,waste edible oil is considered to contain solid foreign impurities,fat/oil degradation (e.g., by deterioration or polymerization) products,or other foreign substances.

However, a predominant portion of a waste edible oil remains unchanged,and if required, the foreign impurities may be removed through anappropriate preliminary treatment such as filtration so as to eliminatethe effects of the impurities. Thus, the method for producing a fattyacid alkyl ester for diesel fuel is applied without any obstacles.

Examples of the alcohol to be reacted with the fat/oil include C1-C3alkyl alcohols such as methyl alcohol (methanol), ethyl alcohol, andisopropyl alcohol, and mixtures of two or more species thereof. Noparticular limitation is imposed on the purity of the alcohol, but a lowwater content is preferred. Among C1-C3 alkyl alcohols, methyl alcoholand ethyl alcohol are more preferred as diesel fuel oil.

No particular limitation is imposed on the reaction method and reactionconditions for the transesterification between fat/oil and alcoholcarried out in the present invention, and any routine methods; e.g.,reaction in the presence of an acid or a base as a catalyst, reaction inthe presence of a solid catalyst (chemical catalyst, bio-catalyst,etc.), and reaction in the absence or in the presence of a microamountof catalyst under high-temperature/pressure conditions (e.g.,subcritical or supercritical state of alcohol) may be employed.Reactions conditions generally employed in the reaction methods may beemployed.

Taking methanol, which is most suitable alcohol, as an example, thealcohol to be reacted with a fat/oil material is preferably employed inan amount of 10 to 30 parts by weight, more preferably 15 to 25 parts byweight, based on 100 parts by weight of the fat/oil, from the viewpointof enhancement of reactivity. Taking into consideration thattransesterification is an equilibrium reaction, the amount of alcohol ispreferably large. However, an amount in excess of 30 parts by weightdoes not enhance reactivity commensurate with increase in amount. Whenan alcohol other than methanol is employed, the aforementionedproportion by weight is reduced to the corresponding equivalentproportion.

Examples of the alkali catalyst employed in transesterification includealkali substances such as sodium hydroxide, sodium carbonate, potassiumhydroxide, potassium carbonate, sodium alcoholates, and potassiumalcoholates. Of these, sodium hydroxide, potassium hydroxide, andsimilar compounds are preferred, with potassium hydroxide being mostpreferably employed in the present invention by virtue of the strongestalkalinity and catalytic action.

Potassium hydroxide is preferably employed in an amount of 0.2 to 1.5parts by weight, more preferably 0.4 to 1.5 parts by weight, based on100 parts of the fat/oil material. The reason for employment of such alarge amount of catalyst is that reactivity of transesterification isrequired to be elevated to as high a level as possible in order toattain a 98% or more of the purity (see the European standards) of thefatty acid methyl ester serving as a fatty acid alkyl ester for dieselfuel, if purification of the fatty acid methyl ester throughdistillation (requiring a large amount of energy) is not taken intoaccount. However, an amount of potassium hydroxide of 1.5 parts byweight or more is not advantageous from the viewpoint of high cost withrespect to the effect thereof.

Furthermore, through employment of a strongly active catalyst such aspotassium hydroxide, the efficiency of transesterification can beelevated, and unreacted triglyceride and diglyceride and monoglyceride(reaction intermediates) can be decreased. Thus, the amount of glycerinwhich is solubilized in the light liquid by diglyceride or monoglycerideis reduced, whereby a load in the light liquid treatment is mitigatedand the amount of water for washing is reduced, thereby decreasingwastes.

The reaction temperature is not a key factor for enhancement ofreactivity, and is preferably 50 to 100° C. from the viewpoint of rateof reaction. Within the range, a higher temperature is more effective.When the temperature in higher than 100° C. and potassium hydroxide isemployed, glycerin by-produced under strong alkaline conditions maypolymerize, or a fat/oil component may undergo decomposition reaction.

<Light Liquid Treatment Step>

The present invention, including transesterification of fat/oil andalcohol, removing excessive alcohol after completion of thetransesterification, separating a heavy liquid predominantly containingglycerin, to thereby produce a light liquid predominantly containing afatty acid alkyl ester, and treatment of the light liquid, ischaracterized in that the treatment of the light liquid comprises

a first step of washing the light liquid with added water;

a second step of causing wash liquid to be absorbed byhigh-water-absorptive resin; and

a third step of removing the high-water-absorptive resin in the form ofgel which has absorbed the wash liquid in the second step, to therebyproduce a fatty acid alkyl ester suitable for use as diesel fuel oil.The first to third steps, included in the light liquid treatment step,will next be described sequentially.

The main purpose of the water-washing step—the first step of the lightliquid treatment step—is to remove a microamount of glycerin, alkalinesubstance, and like substances contained in the light liquid. Anotherpurpose of the water-washing step is to remove a portion of othercomponents; e.g., amphiphilic substances such as diglyceride andmonoglyceride, through partition-dissolution in water for washing orsimilar means.

In an ideal state, the solubility of glycerin in a pure fatty acid alkylester is negligible. In the case of the corresponding methyl ester, thesolubility thereof is about 0.01%, which is lower than a glycerincontent of 0.03% as stipulated in the European standards. However, thefatty acid alkyl ester obtained through transesterification containsglycerin solubilized by amphiphilic substances such as monoglyceride anddiglyceride, which are reaction intermediates. Thus, even though thereactivity is about 98%, the fatty acid alkyl ester contains about 0.07to 0.09% glycerin.

If water-washing is performed in order to remove a microamount ofglycerin and a comparative amount of dissolved alkaline substance, alarge amount of water for washing should not be required. However,because of the presence of monoglyceride and diglyceride, which areamphiphilic substances, addition of a small amount of water to the esterand stirring the mixture results in formation of an emulsion.Particularly when this operation is performed under low-temperatureconditions so as to prevent hydrolysis of the ester, separation of theaqueous phase from the emulsion becomes considerably difficult. In orderto prevent formation of such an emulsion in a conventional method ofwashing with water, a large amount of water must be used. Washing withwater may be performed under such mild stirring conditions that themixture assumes almost two separated layers without forming an emulsion.However, water-washing efficiency is considerably low due to washingemploying a small interface area.

As mentioned above, the conventional methods of washing with water mustemploy a large amount of water so as to prevent emulsification andattain satisfactory washing effect, thereby facilitating separation ofwater after washing. In contrast, the present invention intentionallyemploys effect of washing in an emulsion state, and washing is performedunder vigorous stirring conditions. Even though the system becomes anemulsion, the emulsion is de-emulsified in the second step throughwater-absorption effect of high-water-absorptive resin, and water usedin washing is removed through absorption by high-water-absorptive resin.

As used herein, the term “vigorous stirring” refers to stirring at leastwith such high intensity that the light liquid and water assume anapparently uniform state (emulsion) without causing separation to twolayers, thereby attaining a sufficiently mixed state throughout thesystem. In contrast, when stirring is performed under weak stirringconditions, two layers remain separated from each other and theinterface therebetween does not clearly appear.

According to the present invention, the light liquid is washed withwater under enhanced contact conditions and under such vigorous stirringconditions. Therefore, excellent washing effect can be attained even byuse of a small amount of water.

In this case, a microamount of dissolved alkaline substance is alsoremoved. In order to prevent hydrolysis of a fatty acid alkyl ester inthe above step, the washing step (the first step) is preferablyperformed at 50° C. or lower.

The amount of water for washing the light liquid is 1 to 10 parts byweight, preferably 1 to 5 parts by weight, based on 100 parts of thelight liquid. When the amount of water is 1 part by weight or lower,washing efficiency is lowered.

The second step of the light liquid treatment step is thewater-absorption treatment step, in which a high-water-absorptive resinpowder is added to the light liquid, and the system is stirred so as tocause washing water to be absorbed by the high-water-absorptive resin. Aportion of monoglyceride and diglyceride, which are amphiphilicsubstances, is also removed through adsorption on or dissolution in thesurface of water that has been absorbed by the high-water-absorptiveresin. Therefore, a high-water-absorptive resin powder having a largesurface area is preferably used.

Through vigorous stirring so as to attain a satisfactory mixed state ofthe light liquid dispersion system, de-emulsification and absorption ofwashing with water are completed virtually instantaneously.

The high-water-absorptive resin is added in an amount of 1 to 20 partsby weight, preferably 1 to 10 parts by weight, based on 100 parts byweight of water for washing. When the amount of the resin is 1 part byweight or less, the amount of washing water that is not absorbed by theresin and remains in the light liquid may increase.

No particular limitation is imposed on the type of thehigh-water-absorptive resin employed in the present invention, and anypolymer is preferably employed, so long as the polymer is awater-swellable polymer having a cross-linked structure, predominantlycontaining an acrylate salt and/or starch, and exhibiting a waterabsorption performance about 400 times or higher with respect to purewater and about 50 times or higher with respect to salt-containingwater, based on the self weight of the polymer. Specifically,high-water-absorptive resin used in paper diapers and sanitary goods andcommercial high-water-absorptive resin products serving asmoisture-retaining material for soil, mud-hardener, etc. may be employedin a wide range.

The third step of the light liquid treatment step is a water-absorbedgel filtration step, in which a swollen gel of high-water-absorptiveresin which has been employed in absorption of washing water is removedthrough filtration. When the high-water-absorptive resin exhibits theaforementioned performance and is present in an amount of 1 to 2 partsby weight or more based on the amount of washing water, the swollen gelof the high-water-absorptive resin assumes the form of a large number ofparticles having a stable form, and can be separated from the lightliquid through a simple separation operation such as filtration orcentrifugation. The thus-removed high-water-absorptive resin gel can bereadily disposed of through optionally drying, and treatment in a firingfurnace or a similar apparatus.

In the aforementioned light liquid treatment step of the presentinvention, the first step (water-washing step) and the second step(water-absorption treatment step) may be performed sequentially orsimultaneously. Specifically, the second step may be performed aftercompletion of the first step, or water for washing andhigh-water-absorptive resin may be simultaneously added to the lightliquid.

In the case of simultaneous treatment, a high-water-absorptive resincontaining water or a combination of water and a high-water-absorptiveresin powder is added to the light liquid and the mixture is stirredvigorously, so as to cause water-soluble substances contained in thelight liquid to be absorbed by the hydrous high-water-absorptive resin.Thereafter, the third step (water-absorbed gel filtration step) isperformed.

The simultaneous treatment is more advantageous than the sequentialtreatment, in that the treatment is simplified by performing the firstand second steps in a single process.

Since, in the simultaneous treatment, the time of contact betweenwashing water and the light liquid is short, or a large portion of thewashing water is present in the high-water-absorptive resin gel, theefficiency of dissolution of water-soluble components contained in thelight liquid is inferior to that in the case of the sequentialtreatment. However, if a load of purification is reduced throughsatisfactory enhancement of the transesterification efficiency, theabove insufficient dissolution efficiency is acceptable. In this case,the third step may also be simultaneously performed by causing the lightliquid to pass through a column filled with a high-water-absorptiveresin containing water.

Notably, performing effective purification by only causing the lightliquid to pass through a column requires a sufficient mixed state and asufficient time of contact between the light liquid and thehigh-water-absorptive resin gel swollen by water. In one preferred meansfor satisfying both requirements, a tank for the light liquid and thecolumn are linked to form a loop, in which the light liquid iscirculated at a sufficient flow rate.

According to the method for treating light liquid employed in thepresent invention, a fatty acid methyl ester for diesel fuel having apurity of fatty acid alkyl ester of 98% or higher and a glycerin contentof 0.03% or lower, which are key quality requirements for fuel, can beproduced without installing a waste water treatment facility.

The present invention will next be described in detail by way ofexamples along with experimental results, which should not be construedas limiting the invention thereto.

EXAMPLE

Waste edible oil (1,000 g) and a solution (210 g) of potassium hydroxide(10 g) in methanol were fed into a stirring bath, and the mixture wasallowed to react at 65° C. for 30 minutes. After completion of reaction,methanol was collected through evaporation. The thus-obtained reactionmixture was subjected to phase separation, to thereby obtain 970 g oflight liquid and 150 g of heavy liquid. Water (5 parts by weight) wasadded to the light liquid (100 parts by weight), and the mixture wasmaintained under vigorous stirring (stirring for uniformly mixing) toform emulsion for 5 minutes for 40° C. (in order to prevent hydrolysisof fatty acid methyl ester), to thereby wash the light liquid withwater. Subsequently, high-water-absorptive resin (Aqua Keep 10SH,product of Sumitomo Seika Chemicals, Co., Ltd.) (10 parts by weight,based on 100 parts by weight of washing water) in the form of powder wasdispersed in the washed mixture, and the mixture was maintained for 10minutes under the same conditions as employed in washing with water,followed by heating to 50° C. At this moment, free water was notobserved. The thus-formed gel of the high-water-absorptive resin whichcontains water was removed through filtration by means of filter paper,to thereby collect a transparent, pale brown liquid. Finally, a subtleamount of water and methanol were evaporated out under reduced pressureat 80° C. The product was found to have a fatty acid methyl estercontent and a glycerin content of 98.2% and 0.02% (both weight basis),respectively, as measured through gas chromatography.

Comparative Example 1

To the light liquid (100 parts by weight) produced through the sameprocedure as employed in the Example, washing water (20 parts by weight)was added. The mixture was maintained under weak stirring (such stirringthat two separated layers were maintained and emulsification wasprevented, with a slightly unclear interface) for 15 minutes for 40° C.Subsequently, the mixture was allowed to stand for 15 minutes, and theaqueous layer was removed. The above procedure was repeated, to therebyobtain two aliquots of wash liquid. The thus-obtained liquid wasslightly opaque. The liquid was dehydrated in a manner similar to thatof the Example, to thereby collect a transparent, pale brown liquid. Theproduct was found to have a fatty acid methyl ester content and aglycerin content of 97.8% and 0.05% (both weight basis), respectively,as measured through gas chromatography.

Comparative Example 2

To the light liquid (100 parts by weight) produced through the sameprocedure as employed in the Example, active terra alba (1 part byweight) was added. The mixture was maintained under vigorous stirring(stirring for uniformly mixing) at room ambient temperature for 15minutes. Subsequently, terra alba was removed through filtration, tothereby collect a transparent, pale brown liquid. The product was foundto have a fatty acid methyl ester content and a glycerin content of97.4% and 0.09% (both weight basis), respectively, as measured throughgas chromatography.

1. A method for producing a fatty acid alkyl ester for diesel fuel,including transesterification of fat/oil and alcohol, removing excessivealcohol through evaporation after completion of the transesterification,separating a heavy liquid predominantly containing glycerin, to therebyproduce a light liquid predominantly containing a fatty acid alkylester, and treatment of the light liquid, wherein the treatment of thelight liquid comprises: a first step of washing the light liquid withadded water; a second step of causing wash liquid from the first step tobe absorbed by high-water-absorptive resin; and a third step of removingthe high-water-absorptive resin in the form of gel which has absorbedthe wash liquid in the second step, wherein in said first step ofwashing the light liquid with added water, the washing is performedunder vigorous stirring and the light liquid and added water are in anemulsion without causing separation into two layers, and in the secondstep the emulsion is de-emulsified.
 2. A method for producing a fattyacid alkyl ester for diesel fuel as described in claim 1, whereinremoving of the third step is performed through separation by filtrationor centrifugal separation.
 3. A method for producing a fatty acid alkylester for diesel fuel as described in claim 1, wherein the first andsecond steps are carried out sequentially.
 4. A method for producing afatty acid alkyl ester for diesel fuel as described in claim 1, whereinthe first and second steps or the first to third steps are carried outsimultaneously.
 5. A method for producing a fatty acid alkyl ester fordiesel fuel as described in claim 1, wherein, in the first step, waterfor washing is used in an amount of 1 to 10 parts by weight based on 100parts by weight of the light liquid.
 6. A method for producing a fattyacid alkyl ester for diesel fuel as described in claim 1, wherein, inthe second step, the high-water-absorptive resin is added in an amountof 1 to 20 parts by weight based on 100 parts by weight of the water forwashing.
 7. A method for producing a fatty acid alkyl ester for dieselfuel as described in claim 1, wherein, 100 parts by weight of a fat/oilmaterial and 10 to 30 parts by weight of methyl alcohol are subjected tothe transesterification in the presence of 0.2 to 1.5 parts by weight ofadded potassium hydroxide serving as a catalyst, the reaction isperformed at 50 to 100° C., and a light liquid obtained from thereaction mixture is treated in said treatment.
 8. A method for producinga fatty acid alkyl ester for diesel fuel as described in claim 2,wherein the first and second steps are carried out sequentially.
 9. Amethod for producing a fatty acid alkyl ester for diesel fuel asdescribed in claim 8, wherein, in the first step, water for washing isused in an amount of 1 to 10 parts by weight based on 100 parts byweight of the light liquid.
 10. A method for producing a fatty acidalkyl ester for diesel fuel as described in claim 9, wherein, in thesecond step, the high-water-absorptive resin is added in an amount of 1to 20 parts by weight based on 100 parts by weight of the water forwashing.
 11. A method for producing a fatty acid alkyl ester for dieselfuel as described in claim 10, wherein, 100 parts by weight of a fat/oilmaterial and 10 to 30 parts by weight of methyl alcohol are subjected tothe transesterification in the presence of 0.2 to 1.5 parts by weight ofadded potassium hydroxide serving as a catalyst, the reaction isperformed at 50 to 100° C., and a light liquid obtained from thereaction mixture is treated in said treatment.
 12. A method forproducing a fatty acid alkyl ester for diesel fuel as described in claim2, wherein the first and second steps or the first to third steps arecarried out simultaneously.
 13. A method for producing a fatty acidalkyl ester for diesel fuel as described in claim 12, wherein, in thefirst step, water for washing is used in an amount of 1 to 10 parts byweight based on 100 parts by weight of the light liquid.
 14. A methodfor producing a fatty acid alkyl ester for diesel fuel as described inclaim 13, wherein, in the second step, the high-water-absorptive resinis added in an amount of 1 to 20 parts by weight based on 100 parts byweight of the water for washing.
 15. A method for producing a fatty acidalkyl ester for diesel fuel as described in claim 14, wherein, 100 partsby weight of a fat/oil material and 10 to 30 parts by weight of methylalcohol are subjected to the transesterification in the presence of 0.2to 1.5 parts by weight of added potassium hydroxide serving as acatalyst, the reaction is performed at 50 to 100° C., and a light liquidobtained from the reaction mixture is treated in said treatment.
 16. Amethod for producing a fatty acid alkyl ester for diesel fuel asdescribed in claim 1, wherein said first step of washing the lightliquid with added water is performed at 50° C. or lower.
 17. A methodfor producing a fatty acid alkyl ester for diesel fuel as described inclaim 1, wherein said excessive alcohol is removed through evaporationprior to said treatment of said light liquid.